Multi-branched or star-shaped polymers were first described by Zelinski in U.S. Pat. No. 3,280,084. The Zelinski patent reports coupling polybutadiene anion (generated by addition of butyllithium to butadiene) with 0.02 to 1 part by weight of divinylbenzene to 100 parts of monomer. The resultant star polymer had a polydivinylbenzene core and several identical arms of polybutadiene radiating from the core. The arms could also be either random or block copolymers of styrene and butadiene, wherein the diene is the major component.
U.S. Pat. No. 3,639,517 to Kitchen et al. describes the synthesis of star polymers with arms of different molecular weights. U.S. Pat. No. 3,985,830 to Fetters and Bi details the preparation of star polymers with a nucleus of more than one molecule of divinylbenzene and more than three arms. These polymers were formed by addition of living homopolymers and copolymers of conjugated diene monomers and block copolymers of conjugated diene monomers and monovinylaromatic monomers to divinylbenzene. These unfunctionalized star polymers have been employed as viscosity index (V.I.) improvers for lube oil compositions (see for example, U.S. Pat. Nos. 5,310,490 and 5,310,814).
Star polymers have also been prepared which contain functionality, such as hydroxy groups, at the ends of the arms. For instance, European Patent Application 0632075 disclosed the preparation of star molecules with hydroxyl groups at the terminus of each arm of the star. These polymers were prepared by addition of ethylene oxide to the living anions at the ends of the arms of the star polymer. These functionalized star polymers were useful in making adhesives, sealants, coatings, films and fibers. However, the functionalization reactions described therein are often inefficient, due to the formation of physical gelation phenomena that produce severe mixing problems (see L. Weber, Makromol. Chem., Macromol. Symp., 3, 317 (1986) and U.S. Pat. No. 5,478,899).
SUMMARY OF THE INVENTION
The present invention provides novel protected, functionalized multi-arm polymers as well as their optionally hydrogenated analogues and their optionally deprotected analogues. The star polymers of the invention are prepared by polymerizing monomers using protected functionalized organometallic initiators to form living polymer anions having a protected functional group incorporated into the structure thereof. The living polymer anions are then coupled using a crosslinkable coupling agent to form a living multi-arm polymer having a central core and protected functional groups on the ends of the polymer arms. Additional monomer is then added to grow additional living arms from the central core. These living arms can then be end capped with a suitable functionalizing or capping agent.
In contrast to star polymers of the prior art, the molecular architecture of compounds of the present invention can be precisely controlled. The monomer identity, the monomer composition and molecular weight of the functional arms can be independently manipulated by varying the monomer charged to the initiator. High functionalization of the living polymer anions is achieved, as the functionalizing agents of the current invention do not cause gelation to occur upon their addition to a living polymer. The nature of the functional groups and the protecting groups can be varied simply by changing the protected functional initiator and the functionalizing agent. In addition, the number of polymer arms can be adjusted by varying the nature of the coupling agent and the ratio of living polymer to the coupling agent.
The present invention also provides processes for preparing the polymers of the invention, which can provide efficient functionalization of the living polymer anions.